1. Field of the Invention
The present invention relates to a VCO and filter, and in particular, to a circuit in which the output bandwidth of the filter and output frequency of the oscillator are controlled by the same control bias current signal. This control signal may be generated from a frequency reference formed from standard IC components utilizing the thermal time constant of silicon.
2. Description of the Related Art
Oscillators are well known in the art of solid state electronics. Oscillator circuits producing a constant frequency signal are extremely useful for performing vital system functions such as clocking. Any constant frequency oscillator requires: 1) a source of power; 2) an amplifying device; and 3) some form of resonant circuit to maintain the frequency of the output.
Many solid-state circuits include a separate crystal having an intrinsic vibrational frequency. Such a circuit utilizes the crystal's inherent vibration to generate a constant frequency output signal. Unfortunately, incorporating an external component such as a crystal into an IC creates additional complexity and expense in the manufacturing process.
Therefore, there is a need in the art for a structure formed from standard integrated circuit components that is capable of maintaining the output of an oscillator at a regular frequency.
Filters are also well known in electronics, and find use in a wealth of applications. Many filters include resistive and capacitive elements, with the resistance and capacitance exhibited by these elements determining bandwidth of the filter output.
Unfortunately, there has been long-standing difficulty in producing filters having precise characteristics for use in integrated circuits. This is attributable to the uncertainty in absolute values of resistance and capacitance exhibited by structures created by conventional silicon fabrication processes.
As discussed in detail below in connection with the operation of conventional switched capacitor filter, one way of surmounting this problem is to introduce an external signal of constant frequency into the circuit. However, this again entails substantial additional expense.
Therefore, there is a need in the art for a filter design that can be incorporated into an integrated circuit and which does not depend upon a constant-frequency clock signal provided by an external source.